High power amplifiers (HPAs), such as those often used in wireless communication systems, are intrinsically characterized by gain compression and either phase compression or phase expansion, so they impose amplitude and phase distortion on the RF signals being amplified. Although operating in the relatively linear range of an HPA can minimize these distortions, RF output signal power is correspondingly restricted, and the power restriction is accompanied by low HPA operating efficiency.
Pre-distortion linearizers attempt to compensate for HPA imperfections by “linearizing”—actually pre-distorting—an RF input signal before it reaches the HPA, ideally providing gain expansion to compensate for the HPA gain compression, and phase expansion (or compression) to compensate for the HPA phase compression (or expansion). The subsequent HPA distortions imposed on the pre-distorted signal result in relatively flat gain and phase responses from the linearizer/HPA combination. In this way, linearizers allow a much greater fraction of the HPA's operating range to be utilized with acceptably low distortion, and correspondingly improved efficiency.
Some currently available FET-based linearizers use FETs in circuit configurations that are either incompatible or difficult to implement with the majority of commercially available integrated FET chips. Some allow for gain expansion but ignore consideration of phase expansion or compression; in other cases, both gain and phase compensations are adjustable but not independently. Some linearizers split an incoming RF signal into portions that are separately processed through a fixed arrangement of different microwave functional blocks, and then reassembled. Such “processed signal linearizers” are relatively complex, costly, and do not lend themselves to chip level integration.
What is needed is a linearizer, and method of operating such a linearizer, that makes use of well characterized and/or readily available integrated circuit components in a simple configuration, to provide gain expansion and phase compensation (expansion or compression as desired), preferably offering a convenient means of adjusting circuit parameters and operating conditions to independently adjust the gain and phase compensations according to the characteristics of the HPA to which the linearizer output is delivered.